Έφτιαξα την execute για βάσεις δεδομένων να εκτελεί πολλαπλές εντολές, με διαχωριστικό το ελληνικό ερωτηματικό. Δεν είχα ασχοληθεί πολύ με το θέμα, και είχα αφήσει την execute να μην επιστρέφει τιμές. Ίσως γιατί υπήρχαν άλλες εντολές για ανάκτηση πεδίων από αρχείο από βάση δεδομένων (εδώ λέμε αρχείο το πίνακα σε μια βάση). Όμως το αποφάσισα καθώς διάβαζα για την SQL και χθες το έφτιαξα.
Επειδή χρησιμοποιώ το αντικείμενο ADOdb και με εξ ορισμού τύπου την βάση mdb της Access, η SQL είναι λίγο διαφορετική. Κυρίως στο πώς ορίζουμε πεδία σε πίνακες.
Στο παράδειγμα παρακάτω με ενδιαφέρει να δείξω τη χρήση του NULL. Μπορούμε σε ένα πεδίο να έχουμε NULL τιμή. Αυτό δεν σημαίνει μηδενικού μήκους αλφαριθμητικό (κενό). Με την χρήση της εντολής ΑΝΑΚΤΗΣΗ (RETRIEVE) δεν έχουμε επιστροφή NULL, μετατρέπεται σε κενό αλφαριθμητικό.
Το παράδειγμα υπάρχει στο νέο info.gsb ως baseG
Need new revision 33, version 9.8
\\ remove base if exist in current directory
base "gSQL"
\\ Make a new table and insert values (M2000 use ADOdb with a special connection string)
\\ data taken from example's of MOSH HAMEDANI for SQL, see this video
\\ https://www.youtube.com/watch?v=7S_tz1z_5bA
\\ Create table changed from original to be used here, but the schema is the same.
\\ Maybe integer definition maybe not the same with original int(11).
\\ We use 3 times execute. One return nothing, but the other two return a record set each
\\ recordset saved to stack of values (as top value each time)
\\ so we call subroutine passing just the stack of values, so we read from there.
\\ Inside each subroutine we make some local variables, using a way to link recordset with properties
\\ property fields of recordset used 3 times, to produce different items. One is the actual object, and the
\\ other two by using an index value we get field value (the default property), one as numeric and
\\ the other as string.
\\ So we have to see about NULL. A NULL can exist if table schema allow it. When a field has NULL value
\\ we get it as numeric type of NULL (we can't use it as a value in expressions, always return NULL),
\\ but if we read it as string value we get empty string.
\\ Here we see two rows, one with a NULL phone and one with empty string (which isn't Null)
\\ To examine if we have NULL we have to check the numeric variant of field.
\\ for checking the NULL we have a string function which return "NULL" or the string value,
\\ including empty string.
execute "gSQL", {
CREATE TABLE customers (
customer_id AUTOINCREMENT PRIMARY KEY,
first_name VARCHAR(50) NOT NULL,
last_name VARCHAR(50) NOT NULL,
birth_date date DEFAULT NULL,
phone VARCHAR(50) DEFAULT NULL,
address VARCHAR(50) NOT NULL,
city VARCHAR(50) NOT NULL,
state char(2) NOT NULL,
points integer NOT NULL DEFAULT 0
);
INSERT INTO `customers` VALUES (1,'Babara','MacCaffrey','1986-03-28','781-932-9754','0 Sage Terrace','Waltham','MA',2273);
INSERT INTO `customers` VALUES (2,'Ines','Brushfield','1986-04-13','804-427-9456','14187 Commercial Trail','Hampton','VA',947);
INSERT INTO `customers` VALUES (3,'Freddi','Boagey','1985-02-07','719-724-7869','251 Springs Junction','Colorado Springs','CO',2967);
INSERT INTO `customers` VALUES (4,'Ambur','Roseburgh','1974-04-14','407-231-8017','30 Arapahoe Terrace','Orlando','FL',457);
INSERT INTO `customers` VALUES (5,'Clemmie','Betchley','1973-11-07',NULL,'5 Spohn Circle','Arlington','TX',3675);
INSERT INTO `customers` VALUES (6,'Elka','Twiddell','1991-09-04','312-480-8498','7 Manley Drive','Chicago','IL',3073);
INSERT INTO `customers` VALUES (7,'Ilene','Dowson','1964-08-30','615-641-4759','50 Lillian Crossing','Nashville','TN',1672);
INSERT INTO `customers` VALUES (8,'Thacher','Naseby','1993-07-17','941-527-3977','538 Mosinee Center','Sarasota','FL',205);
INSERT INTO `customers` VALUES (9,'Romola','Rumgay','1992-05-23','559-181-3744','3520 Ohio Trail','Visalia','CA',1486);
INSERT INTO `customers` VALUES (10,'Levy','Mynett','1969-10-13','','68 Lawn Avenue','Atlanta','GA',796);
}
\\ id 10 has no Null phone, has no phone (is an empty string)
\\ Null means "no information yet"
report "customers created"
\\ execute a SE`LECT query to get a Recordset
report "Selected customers where Points<1000 in descending order"
execute "gSQL",{
SELECT customer_id,
last_name,
first_name,
phone,
points
FROM `customers`
WHERE Points<1000
ORDER BY Points DESC,
last_name,
first_name
}
Def CheckNull$(a, a$)=If$(type$(a)="Null"->"NULL", a$)
GetListA() ' value is already in stack
report "Select customers id where phone is null"
execute "gSQL",{
SELECT customer_id,
phone
FROM `customers`
WHERE phone IS Null
ORDER BY customer_id
}
GetIDwithNullPhone() ' value is already in stack
\\ using list to check the variable names
list
\\ there are no variables in module, at this moment.
\\ All variables was local to subs, and they erased at the exit of subs
\\ we can use dots in names, like a normal variable.
\\ so rs.fields is a norma variable name. This variable is a PropReference type.
\\ This type of variable is an object which link a property to an object
\\ We can't use these variables as return values, because the use a hard link,
\\ and not a standard reference to object. So PropReference can't hold alive a linked object.
\\ So if we return the PropReference from where we make it then became invalid, and we have
\\ to use it to raise the error.
sub GetListA(RS)
if type$(RS)="Recordset" then
with RS, "EOF" as new rs.eof, "fields" as new fields(), "fields" as new fields$(), "fields" as new rs.fields
with rs.fields, "count" as new rs.fields.count
print "Test number of fields:", rs.fields.count
while not rs.eof
print $(6),fields(0),
print $(9)," ";fields$(1);" ";fields$(2);@(32); CheckNull$(fields(3), fields$(3));@(42), fields(4)
method rs, "movenext"
end while
print $(0),,
end if
end sub
sub GetIDwithNullPhone(RS)
if type$(RS)="Recordset" then
with RS, "EOF" as new rs.eof, "fields" as new fields(), "fields" as new fields$(), "fields" as new rs.fields
with rs.fields, "count" as new rs.fields.count
print "Test number of fields:", rs.fields.count
while not rs.eof
print $(4), fields(0), $(7), CheckNull$(fields(1), fields$(1)), $(0)
method rs, "movenext"
end while
print
end if
end sub
